1. Field of the Invention
This invention relates to a method for surface treatment which forms a layer of molybdenum (Mo) carbonitride on the surface of any articles made of iron or an iron alloy, such as dies, jigs, tools and machine parts.
2. Description of the Prior Art
Carbides of molybdenum (Mo), such as Mo.sub.2 C and (Mo,Fe).sub.6 C, has a hardness of more than Hv 1500 and have superior resistance to wear and seizure than that of the carbide of iron (Fe.sub.3 C) or the nitride of iron (Fe.sub.2.about.3 N). Molybdenum carbide has been made to exist in high speed steel in the form of (Mo,Fe).sub.6 C to improve wear resistance in addition to hardness. However, the carbide of Mo has a lower hardness and a poorer wear resistance than those of the carbide of V, Ti or the like, having a hardness of about Hv 3000 and, therefore, there have been only few practical uses thereof for a wear resistant coating layer. In addition, MoN is also poor in wear resistance compared with VN, TiN. Although MoS is an excellent solid lubricant, the seizure resistance of the carbide and nitride of Mo has not sufficiently been examined. The inventors of this invention have found that the carbonitride of Mo exhibits an excellent seizure resistance, and they conceived of forming a surface layer composed of the carbonitride of Mo on the surface of iron or an iron alloy article (hereinafter referred to as an article to be treated) thereby to improve the properties of the article to be treated.
In a conventional method for coating the carbide of molybdenum, the iron alloy article is immersed in a molten salt bath composed of the chloride system to form a layer of the carbide of molybdenum on the surface of the article.
According to the above method, however, the article is heated at a temperature which is higher than the A.sub.c1 transformation point of iron, which is about 700.degree. C. The heat is likely to develop (in the article) a stress which causes it to crack if it has a complicated shape. Moreover, it worsens the working environment, because treatment is done at high temperatures.
To form a surface layer containing molybdenum, there have also been proposed methods which employ a temperature which is lower than about 700.degree. C. They include CVD (chemical vapor deposition) and PVD (physical vapor deposition) employing halides of molybdenum. It is, however, difficult to form by any of those methods a layer having a uniform thickness and adhering closely to the surface of the article. They involve a complicated process which requires expensive facilities. Moreover, they require the presence of hydrogen or a reduced pressure which lowers the efficiency of the operation.